The Philippines' most active volcano belched out choking ash Tuesday as authorities, fearing a major eruption, struggled to deal with thousands of evacuees crammed into shelters.

The ash spread over a wide terrain by Mount Mayon, about 330 kilometres (200 miles) southeast of Manila, raised new health fears for locals already braced for an eruption that scientists say could come any day.

More than 45,000 evacuees taking refuge in overcrowded buildings were set to endure Christmas away from home, officials said, as authorities vied to bring in relief supplies and prevent outbreaks of disease.

Vehicles driving along roads around Guinobatan village, some 14 kilometres from Mayon, kicked up clouds of volcanic dust as residents complained of stinging eyes and irritated skin.

"It's not very thick, just a few millimetres of ash, but that is the most dangerous part because it is very fine ash," chief government volcanologist Renato Solidum said in a television interview.

Health officials warned the tiny particles could cause respiratory problems or skin diseases, and could affect the evacuees sheltering beyond the eight-kilometre danger zone.

Mayon, which has been spewing lava and ash for a week, remains at alert level four, meaning it could violently erupt any day.

The elevated risk has prompted authorities to evacuate more than 9,440 families -- at least 45,336 people -- from the danger zone.

Provincial governor Joey Salceda said there were only about 500 people still in the zone, where some farmers have been reluctant to abandon their holdings in the fertile pastures around Mayon.

"We are doing all sorts of contortions just to bring them in," Salceda told AFP, while adding he was reluctant to resort to force.

He said "definitely (evacuated villagers) will spend Christmas and even New Year's Day in evacuation centres".

In some school buildings being used as shelters, whole families were packed into rooms where they had to sleep and eat on the floor or on top of school desks.

Authorities were rushing to install additional toilets and bring in more clean water to avert disease, provincial disaster control operations officer Jukes Nunez said.

"We are running out of resources, especially rice supplies, for distribution to the evacuees and with the cold of the season, we need more blankets and sleeping mats," he told AFP.

Government doctors have been sent to the centres to check on the most vulnerable evacuees, including the elderly and children.

Governor Salceda said he feared that bored evacuees may try to sneak out and return home so they can celebrate Christmas in familiar surroundings.

To avert this, activities are being laid on in the evacuation centres such as concerts, painting lessons and children's parties, he said.

Witnesses reported ashfall several kilometres from the restive volcano and civil defence officials said it was hard to predict where the ash would settle because of shifting winds.

Volcanic ash has proved deadly in the past. During Mayon's last eruption in 2006, the volcano oozed lava and vented steam and ash for two months.

No one was killed by the eruption itself, but three months later, a powerful typhoon dislodged tons of volcanic ash and debris that had collected on Mayon's slopes. The fast-moving avalanche of mud and boulders crushed entire villages, leaving more than 1,000 people dead.

The 2,460-metre (8,070-foot) volcano, which is famed for its near-perfect cone, has erupted 48 times in recorded history. In 1814, more than 1,200 people were killed as lava buried the town of Cagsawa.